On 19.03.21 00:53, Balbir Singh wrote:
On Thu, Mar 18, 2021 at 05:57:06PM +0100, Vlastimil Babka wrote:
On 3/11/21 7:14 PM, David Hildenbrand wrote:
Hi folks,
I was wondering, is there any mechanism that reclaims basically empty page
tables in a running process?
Like: When I MADV_DONTNEED a huge range, there could be plenty of basically
empty (e.g., all entries invalid) page tables we could reclaim. As soon as we
zap a complete PMD we could reclaim (depending on the architecture) a whole page.
Zapping on the PMD level might make most impact I guess.
For 1 GB, we need 262144 4k pages. If we assume each PTE is 8 bytes, we need a
total of 8 MB for the lowest level page tables (PTE).
OTOH, we would need 512 PMD entries - a single 4k page. Zapping 1 TB would mean
we can free up another 4MB - rather a corner case and we can live with that.
Of course, the same might apply to other cases where we can restore all page
table content from the VMA again. One example would be after MADV_FREE zapped a
whole range of entries we marked.
I don't think we have such mechanism, but IIRC I've heard the idea mentioned
before, probably from Michal Hocko. Definitely an interesting research project
idea to evaluate the cost vs benefits of that.
It might lead to interesting interactions with lockless page table walking
with implications on the mmap_lock as well.
I think if lockless page table walks have to be able with THP code
swapping populated page tables by a PMD back and forth, swapping an
unpopulated page table by an invalid PMD entry might be quite similar.
At least it feels like both approaches would rely on similar mechanisms
/ locking. :)
I'm planning on looking into this, but not sure when I'll have time to
prototype something up.
Balbir
--
Thanks,
David / dhildenb
